DIAGNOSIS The hub is online (you can see it in the app), but it's lost the signal from the door sensor. The little white sensor on the door frame and the magnet on the door itself have gotten misaligned or the battery is dead.
FIX First, check the alignment. The sensor has a little LED. Open the garage door manually. When the door is fully open, the LED should be solid. Close it. When fully closed, it should also be solid. If it's blinking or off, the magnet isn't close enough. Loosen the sensor bracket and slide it so it's directly across from the magnet (within a half-inch). Tighten it back up.
IF THAT DIDN'T WORK The sensor uses a CR2032 coin battery. Even if it looks aligned, pop the cover off and replace the battery. After replacing, you'll need to re-pair it by pressing the button on the back of the sensor until the LED blinks, then press the button on the hub.
HEADS UP Metal garage door tracks or frames can sometimes interfere with the magnetic signal. If alignment is perfect and battery is new, try moving the sensor mount a few inches up or down the frame, away from the metal track.